Methods of solution mining uranium have been perfected to the extent that the processes are now commercial. In-situ leaching of uranium mineralized in unconsolidated sandstone layers underground is typically carried out using an alkaline bicarbonate such as an ammonium bicarbonate-ammonium carbonate solution along with an oxidant such as a peroxide. The sandstone stata amendable to in-situ leaching is generally contained between confining shale or mudstone layers. The mineralization also generally contain some amount of clay usually in the range 5 to 15%, consisting predominantly of montmorillonite or kaolinite with small amounts of illite and clinoptilolite clays.
The uranium that is leached from the in-situ ore body and is in the pregnant leach solution is recovered by hydrometallurgical methods, such as ion exchange. The leach solution is recycled after restoring its chemical strengths in the reagent and the oxidant. During the in-situ leaching process due to the underground mineralization, the ammonium ion in the leach solution may be absorbed by the clay fraction. The counter ion released from the clay may be calcium, magnesium, sodium, potassium, etc., depending upon the state of the virgin clay. One of the principal problems in the utilization of this technology is the restoration of the leached mineralization to a stable aquifer as mandated by state and federal legislation. Until now, techniques for accomplishing the restoration of the aquifer were limited to pumping old water out of the aquifer and discarding it and permitting ground water to seep into the mine zone from the surrounding aquifer, a technique known as ground water sweep. Another technique that was also used was called "clean water recycle" which involved cleansing the solution pumped from the aquifer through a reverse osmosis membrane and pumping the cleansed water back underground until the ammonium ions were removed from the aquifer. Both these techniques could require long periods of time to reduce the ammonium ion level to acceptable levels depending on the characteristics of the clay.